A Global Positioning System (GPS) is known as a system that can be used for measuring a position of a terminal. Currently about 20 GPS satellites are in operation, each of which transmits a navigation message(s). As shown in FIG. 10, a navigation message contains various data such as, time correction data, ephemeris data and almanac data. Ephemeris data from a GPS satellite indicates a precise orbit of the GPS satellite, and almanac data indicates approximate orbits of all GPS satellites.
As shown in FIG. 11, GPS satellites move in different orbits around the earth. Consequently, a terminal 1 is not able to simultaneously receive radio signals from all GPS satellites. Namely, a radio signal that the terminal is able to receive from a GPS satellite is dependent on a current position of the terminal.
In the example shown in FIG. 11, terminal 1 is able to receive radio signals from GPS satellites 4-2 to 4-5 when it is located in area 5-1; and is able to receive radio signals from GPS satellites 4-3 to 4-6 when it is located in area 5-2. These GPS satellites from which terminal 1 can receive radio signals are referred to as “GPS satellites in the visible area” for terminal 1.
A terminal 1 with GPS function receives radio signals from two or more GPS satellites respectively, computes distances from the GPS satellites on the basis of the navigation messages included in the radio signals, and thereby measures the position of the terminal. This measurement operation will be referred to as a “position measurement operation.”
A specific measurement operation carried out by terminal 1 will now be described below referring to FIG. 11, taking a case where terminal 1 is positioned in area 5-1 as an example. First, terminal 1 receives navigation messages from one of the GPS satellites 4 (e.g. GPS satellite 4-2), and extracts almanac data from the received navigation messages.
Next, terminal 1 determines the GPS satellites in the visible area for terminal 1 on the basis of the almanac data. In this example, GPS satellites 4-2 to 4-5 are determined as the GPS satellites in the visible area for terminal 1.
Next, terminal 1 tunes into GPS satellites 4-2 to 4-5 in the visible area, and receives navigation messages from GPS satellites 4-2 to 4-5, respectively.
Then terminal 1 computes distances between terminal 1 and each one of the satellites 4-2 to 4-5 on the basis of ephemeris data included in the navigation messages, and measures the position of the terminal.
Each GPS satellite transmits a navigation message at a transmission rate of 50 bps. The navigation message consists of 25 frames as shown in FIG. 10, while each frame consists of 1,500 bits. Therefore, a time of 750 seconds (1,500 bits×25/50 bps) is required to receive the complete navigation message. Thus, a time taken by terminal 1 to receive a navigation message is relatively long at the commencement of a position measurement operation; and terminal 1 must then extract almanac data from the navigation message. As a result, terminal 1 suffers from a drawback in that it is not convenient to use since a position measurement operation that it carries out takes a relatively long time.